Ordinarily, you hear sounds when vibrations traveling through the air get captured by the outer ear (or pinna) and sent to the inner ear (or cochlea), where they are transformed into nerve impulses and sent to the brain.
Here, sound waves from a vibrating rod travel directly to your inner ear via bone conduction—sound vibrations travel through your teeth and bones when you bite down on the rod.
Sound travels much more efficiently through solids than through air. A person biting the rod can hear music loud and clear, others standing nearby don’t hear a thing.
This early hearing aid, called a Dentaphone, worked by bone conduction; using it required biting down on a sheet of hardened rubber. Some modern hearing aids still rely on bone conduction—but through the skull, not the teeth. Photo courtesy of Becker Medical Library, Washington University School of Medicine. (click image to enlarge)